The invention relates to a method and a device, in particular a control-and-evaluation unit, for operating at least one exhaust-gas sensor for monitoring the functionality of an exhaust emission control system in the exhaust train of an internal-combustion engine, wherein the exhaust-gas sensor is operated at high temperatures at least temporarily and, due to its type of construction, exhibits a sensitivity to thermal shock, and in which prior to a regeneration phase or prior to a measuring-mode phase a heating phase can be carried out at least temporarily, wherein in this heating phase a distinctly lower temperature is adjusted in comparison with the regeneration temperature or measuring-mode phase.
The invention further relates to a device, in particular a control-and-evaluation unit for operating exhaust-gas sensors and for implementing the method as claimed in the invention.
Exhaust-gas sensors—such as, for example, lambda probes, particle sensors, HC sensors or nitrogen-oxide sensors—as claimed in the state of the art are based on ceramic sensor elements that in operation are heated at least temporarily.
Particle sensors (PM) are employed nowadays, for example, for monitoring the particulate-matter (soot) emission of internal-combustion engines and for on-board diagnosis (OBD), for example for functional monitoring of particle filters, for example of a diesel particle filter (DPF). Such a resistive particle sensor is described in DE 101 33 384 A1. The particle sensor has been constructed from two intermeshing, comb-like electrodes (interdigital electrodes IDE) which have been covered at least partly by a trapping sleeve. If particles from a stream of gas are deposited on the particle sensor, this results in an evaluable change in the impedance of the particle sensor, from which the quantity of accreted particles—and consequently the quantity of particles entrained in the exhaust gas—can be inferred.
If the particle sensor is fully laden, the accreted particles are burnt in a regeneration phase with the aid of a heating element integrated within the particle sensor. For this purpose, the ceramic of the sensor element is heated to high temperatures, ordinarily to >600° C. As a rule, the heating element exhibits a temperature-measuring structure (meander) with which the temperature of the particle sensor can be monitored and the heating power during this regeneration phase can be regulated.
In this regeneration phase the sensor element reacts sensitively to great local changes of temperature or to a thermal shock such as may arise as a result of incident water or drops of water. A thermal shock of such a type can lead to cracks in the sensor element. Similar problems also arise in the case of the other aforementioned exhaust-gas sensors. Therefore a sensor regeneration is demanded by the engine control unit only when, as claimed in a heat-quantity calculation in the engine control unit, no more water can be present at the installation position of the sensor.
Furthermore, a loading with water must be prevented from occurring while the temperature of the sensor element of exhaust-gas sensors of such a type is greater than a certain threshold-value temperature, typically about 200° C. Therefore, particularly after a cold start, as long as condensate may still be located in the exhaust train of an internal-combustion engine, operation with heating at a temperature >200° C. of the exhaust-gas sensors takes place only after a certain time in which it may be assumed that, in this time, all the water has either evaporated or been discharged, in the form of droplets, from the exhaust train by virtue of surges of gas in automotive operation. This moment is typically designated as the dew-point end (DPE) and depends on many conditions, for which reason this moment has to be determined in the specific application for each type of vehicle. Operation of the exhaust-gas sensors at temperatures >200° C. is then permitted, so long as no condensation of water in the exhaust train occurs in the region of the installation point of the sensor as a consequence of cooling.
Particle sensors and also other heated exhaust-gas sensors utilize known dew-point detection functions in order to detect whether liquid water is present in the exhaust train (see, for example, document DE 43 00 530 C2 or DE 43 38 342 C2). With these functions it is determined, on the basis of the exceeding of a pipe-wall temperature threshold and on the basis of the exceeding of a threshold quantity of exhaust-gas heat, that no more liquid water can be present at the location of the sensor. The limiting quantity of heat is, in turn, dependent on the pipe-wall temperature that was present at the start of the driving cycle. The temperatures taken as a basis for this—such as the exhaust-gas temperature and the pipe-wall temperature—are model-based in the contemporary practical realization and are reproduced in an exhaust-gas-temperature model.
As a result of intense loading of the exhaust system with water from outside, for example during fording travel or in the course of launching a boat into water along a slip-ramp, flooding may occur, whereby water penetrates into the exhaust train, as a result of which an intense cooling of the exhaust system may occur. Depending on the configuration of the exhaust system, this cooling cannot be detected by the engine control unit but may result in an exposure to danger as a result of thermal shock to the sensor element of the exhaust-gas sensors. This case has not been covered hitherto in a dating of the dew-point end—that is to say, water remains in the exhaust train, even though the previous DPE-detection function cannot detect this on the basis of limiting quantities of heat and pipe-wall temperatures. If the sensor is regenerated while or after this state has arisen, the risk of a thermal shock is particularly high.
Furthermore, also during fording travel the exhaust pipe may be cooled in such a manner that a condensation of liquid water occurs. The sensor is then loaded with drops of liquid. The dew-point detection function is frequently based on model temperatures (see, for example, DE 10 2006 010 094 A1) which, in turn, are based on temperature measurements that were carried out using sensors which, however, have been arranged upstream of the particle sensor and the aforementioned exhaust-gas sensors. Since these temperature sensors remain unaffected by the fording state, such a fording state cannot be detected. If the particle sensor is regenerated, for example, damage to the sensor may occur as a result of thermal shock, even though a dew-point end was detected.
Furthermore, an increase may occur in the IDE current signal, which is caused by the conductivity of liquid water. This may be misinterpreted as soot loading of the sensor element.
In other, as yet unpublished, parallel applications by the applicant—inter alia, for example, the application having internal file reference R.351847(corresponding to DE 1020 13223429)—fording-detection criteria and also flood-detection criteria are furthermore described. Furthermore, in the likewise as yet unpublished application by the applicant having internal file reference R.347466 (corresponding U.S. Pat. No. 9,377,425) a utilization of a combination of criteria for enabling the heating of the exhaust-gas sensor is also described.